Systems and methods for reducing narrow bandwidth interference contained in broad bandwidth signals

ABSTRACT

Under one aspect of the present invention, a system for processing a group of signals and interference includes (a) an analog-to-digital (A/D) converter for digitizing the group of signals and the interference; (b) a Fourier transform circuit for obtaining a Fourier transform of the digitized group of signals and the interference and to provide as output spectral bins, at least one of which contains the interference; (c) a power analysis circuit for comparing the collective power level of the spectral bins to a predetermined threshold, and if the collective power level exceeds the predetermined threshold, and for excising at least one bin that contains the interference; and (d) an inverse Fourier transform circuit for obtaining an inverse Fourier transform of the remaining spectral bins and outputting a digitized group of signals less the interference in any excised spectral bin.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 U.S.C. §120 of U.S. patentapplication Ser. No. 13/676,645, filed Nov. 14, 2012 and entitled“Systems and Methods for Reducing Narrow Bandwidth InterferenceContained in Broad Bandwidth Signals,” the entire contents of which areincorporated by reference herein.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under Contract No.FA8802-09-C-0001 awarded by the Department of the Air Force. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

This application generally relates to systems and methods for reducinginterference contained in signals.

BACKGROUND OF THE INVENTION

A receiver may be configured to receive and process signals that havebroad bandwidth spectra and powers within a certain, expected, range.For example, a receiver on a satellite may be configured to receive agroup of signals that share a common region of the electromagneticspectrum, and are multiplexed with one another using techniques known inthe art. In the multiplexing technique known as code division multipleaccess (CDMA), each signal of the group is encoded with a unique code,and spread over the same selected portion of the spectrum as the othersignals in the group. The receiver receives the group of signals, andthen decodes one or more of the signals from others in the group using apriori knowledge about the unique code(s) of those signals.Alternatively, in the multiplexing technique known as frequency-divisionmultiple access (FDMA), each signal of the group is assigned a differentsub-portion of the region of the spectrum than the other signals in thegroup. The receiver receives and processes the group of signals, andthen differentiates one or more of the signals from others in the groupusing a priori knowledge about the spectral sub-portion(s) of thosesignals. The groups of signals received in both CDMA and FDMA may beconsidered “broad bandwidth” signals because the groups of signalsoccupy a portion of the electromagnetic spectrum that is significantlybroader than normally would be used for a single, non-multiplexedsignal, that is, a “narrow bandwidth” signal.

In both CDMA and FDMA, the overall power of the group of signalsreceived by the receiver preferably is sufficiently higher than anynoise sources that may be present to yield a sufficient signal-to-noiseratio (SNR) to communicate signals with adequate fidelity as measured byBER (Bit Error Rate) values. At the same time, the overall power of thegroup of signals also preferably is sufficiently low that the receivermay process the signals without distortion. Specifically, as is known inthe art, receivers have a linear range of operation and a nonlinearrange of operation. If a signal input to the receiver has a power thatfalls within the linear range of the receiver, then the receiverprocesses the received signal collection without distortion. However, ifa signal input to the receiver has a power that falls within thenonlinear range of the receiver, then the received signal collection isdistorted and communication performance is degraded.

Signals other than the desired group of signals that the receiverreceives may be referred to as “interference,” may be intentional orunintentional, and may have a broad bandwidth or a narrow bandwidth. Ifthe receiver receives interference that falls within the same portion ofthe electromagnetic spectrum as the desired group of signals, then thereceiver may not distinguish the interference from the group of signalsagain degrading communication performance. However, if the power of theinterference is sufficiently high that nonlinear receiver operationoccurs, not only may the interference obscure desired spectralcomponents but also cause additional signal distortion. This additionalreceiver distortion may include suppression of desired signals andgeneration of intermodulation products between design signal componentsand the interference, resulting in additional degradation in receiverperformance.

A receiver may have features intended to reduce the effects of suchinterference. For example, the receiver may be designed so as toincrease its linear range, and thus reduce the risk that interferencemay cause distortion, e.g., by providing circuitry that remains linearat higher input power levels. However, increasing the linear range ofthe receiver may be expensive, and also may require a larger powersupply to operate the modified circuitry.

Another known approach for reducing the effect of narrow bandwidthinterference on reception of a broad bandwidth desired signal usesadaptive notch filter techniques. Specifically, a notch filter may beapplied to the received signal prior to amplification so as to block theregion of the spectrum where the interference is located. The amplitude,width, and spectral location of the notch filter may be adaptivelymodified over time by varying weighting coefficients, which may beiteratively derived using a gradient process based on an optimizationcriterion, such as maximum signal to noise plus interference ratio(SNIR). Such adaptive notch filter techniques have been widely applied.However, its iterative nature makes this approach is relatively slow,and thus less able to respond to rapidly changing interference.

The CDMA signal format is an example of spread spectrum modulationwherein user signals are spread over a much wider bandwidth than neededto convey the information in the user's signal. One advantage of spreadspectrum modulation is protection from interference achieved byprocessing the user-unique codes. Similar interference protection may beachieved in FDMA formats by frequency hopping the user assignedfrequency slots over a wide bandwidth in a pseudorandom sequence offrequency hop codes known to both the sender and receiver. Signal errorcorrecting coding and interleaving techniques further add to theinterference protection and are commonly used. These interferenceprotection techniques are known in the art, but their benefits depend onlinear receiver operation. The effectiveness of these techniques issignificantly degraded by receiver nonlinearities.

Thus, what is needed is a method of reducing the effects of interferencewith broad bandwidth signals while maintaining linear receiveroperation.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide systems and methods forreducing narrow bandwidth interference contained in broad bandwidthsignals. Specifically, embodiments of the present invention removerelatively high-power, narrow bandwidth interference contained in abroad bandwidth, desired signal after the composite of the two arereceived, for example by obtaining a Fourier transform of the compositesignal and then identifying and excising spectral regions that haveparticularly high power levels. Following such excision, an inverseFourier transform may be obtained, the result of which is a signalhaving reduced interference. Preferably, the spectral regions to beexcised are selected based on the overall power levels of the broadbandwidth desired signal, so as to maintain such power levels within thelinear operating range of electronic components to which the processedsignal subsequently may be provided.

Under one aspect of the present invention, a system for processing agroup of signals and interference includes (a) an analog-to-digital(A/D) converter configured to digitize the group of signals and theinterference; (b) a Fourier transform circuit coupled to the A/Dconverter and configured to obtain a Fourier transform of the digitizedgroup of signals and the interference and to provide as output spectralbins, each bin having a power level, at least some of the binsrespectively containing portions of the digitized group of signals, andat least one of the bins containing the interference; (c) a poweranalysis circuit configured to compare the collective power level of thespectral bins output by the Fourier transform circuit to a predeterminedthreshold, and if the collective power level exceeds the predeterminedthreshold, and to excise at least one bin that contains theinterference; and (d) an inverse Fourier transform circuit configured toobtain an inverse Fourier transform of the remaining spectral bins andto provide as output a digitized group of signals in the time-domainless the interference in any excised spectral bin.

In some embodiments, the A/D converter and Fourier transform circuit areconfigured to ensure that the group of signals and the interference arewithin the linear range of these components, e.g., fall within the A/Dconverter's digital quantization level. In some embodiments, the systemalso includes an analog conditioner circuit configured to ensure thatthe group of signals and the interference are within the conditioner'slinear range.

In some embodiments, the power analysis circuit is configured to obtaina dynamically defined threshold having a value that, if spectral binshaving power levels exceeding that threshold are excised, would reducethe collective power to or below the predetermined threshold. The poweranalysis circuit may be configured to excise spectral bins having powerlevels that exceed the dynamically defined threshold. For example, thepower analysis circuit may include a first arithmetic circuit configuredto obtain the collective power level, a storage medium configured tostore the predetermined threshold, and a first comparator configured tocompare the collective power level to the stored predeterminedthreshold. The power analysis circuit further may include a secondarithmetic circuit configured to obtain the dynamically definedthreshold, and a second comparator configured to compare the power levelof each bin to the dynamically defined threshold and to excise any binwhose power level exceeds that threshold. In some embodiments, the poweranalysis circuit includes a field programmable gate array (FPGA) orapplication specific integrated circuit (ASIC) suitably programmed so asto include the first and second arithmetic circuits, the storage medium,and the first and second comparators.

The power analysis circuit may be configured to excise the spectral binshaving power levels that exceed the dynamically defined threshold bysetting the power levels of those bins to zero.

In some embodiments, the power analysis circuit further also may beconfigured to compare the power level of each spectral bin to an emptybin threshold, and if the power level of a bin is less than the emptybin threshold, excising that bin.

The group of signals may include code-division multiple access (CDMA) orfrequency-division multiple access (FDMA)-based signals, for example.

Some embodiments further include a multiplexer configured to multiplexthe digitized group of signals output by at least one antenna, and atleast one router configured to route the multiplexed signals to the atleast one antenna based on the intended destination thereof. In thiscase, the receiver may be a part of a transponder that relays a group ofsignals to another location(s), e.g., such as the transponder in acommunication satellite or wireless network. In other embodiments, thereceiver may be equipped with demodulator(s) to obtain the informationcontained in one or more signals, e.g., as would be the case whereindividual system users demodulate signals of interest for their ownpurposes.

Under another aspect of the present invention, a method of processing areceived group of signals and interference includes (a) digitizing thereceived group of signals and the interference; (b) obtaining a Fouriertransform of the digitized group of signals and the interference tooutput spectral bins, each bin having a power level, at least some ofthe bins respectively containing portions of the digitized group ofsignals, and at least one of the bins containing the interference; (c)comparing the collective power level of the spectral bins to apredetermined threshold, and if the collective power level exceeds thepredetermined threshold, excising at least one bin that contains theinterference; and (d) obtaining an inverse Fourier transform of theremaining spectral bins and providing as output a digitized group ofsignals less the interference in any excised spectral bin.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A illustrates a system for reducing interference in a receiver,according to some embodiments of the present invention.

FIG. 1B illustrates a system for reducing interference in a transponder,according to some embodiments of the present invention.

FIG. 2 illustrates a method of reducing interference in a receiver,according to some embodiments of the present invention.

FIG. 3A illustrates an exemplary group of code division multiple access(CDMA) signals and interference that may be received by theantenna/analog conditioner of a receiver.

FIG. 3B illustrates an exemplary set of spectral bins followingdigitization and Fourier transformation of the group of signals andinterference of FIG. 3A at an instant of time during portion eillustrated in FIG. 3A.

FIG. 3C illustrates the exemplary set of spectral bins of FIG. 3Bfollowing excision of bins containing interference.

FIG. 3D illustrates the instantaneous power spectral distribution duringthe instant of time during portion e illustrated in FIG. 3A.

FIG. 3E illustrates an exemplary group of CDMA signals, lessinterference in the bins whose power the power analysis circuit set tozero as illustrated in FIG. 3C, following inverse Fouriertransformation.

FIG. 4A illustrates an exemplary group of code division multiple access(FDMA) signals and interference that may be received by theantenna/analog conditioner of a receiver, prior to digitization.

FIG. 4B illustrates an exemplary set of spectral bins followingdigitization Fourier transformation of the group of signals andinterference of FIG. 4A at an instant of time during portion fillustrated in FIG. 4A.

FIG. 4C illustrates the exemplary set of spectral bins of FIG. 4Bfollowing excision of bins containing interference. FIG. 4D illustratesan exemplary group of FDMA signals, less interference in the bins whosepower the power analysis circuit set to zero as illustrated in FIG. 4C,following inverse Fourier transformation.

FIG. 5 illustrates an alternative transponder, according to someembodiments of the present invention.

FIG. 6 illustrates a digital channelizer, according to some embodimentsof the present invention.

FIG. 7A illustrates a digital combiner, according to some embodiments ofthe present invention.

FIG. 7B illustrates signals that may be combined using the digitalcombiner of FIG. 7A.

FIGS. 8A-8B illustrate exemplary groups of signals having variablebandwidths.

FIG. 9A illustrates a simulated CDMA signal with narrow bandwidthinterference at two locations.

FIG. 9B illustrates the simulated CDMA signal of FIG. 9A followingexcision of the narrow bandwidth interference.

FIG. 10 illustrates the bit error rate (BER) for the signals of FIGS. 9Aand 9B.

DETAILED DESCRIPTION

Embodiments of the present invention provide systems and methods forreducing narrow bandwidth interference with broad bandwidth signals,such as a group of signals that are multiplexed with each other and thatshare a common portion of the electromagnetic spectrum. As describedabove, if a receiver receives both desired broad bandwidth signals andhigh level narrow bandwidth interference, the composite of the signalsand interference may result in nonlinear receiver operation distortingthe desired broad bandwidth signals, thus reducing the receiver'sability to retrieve information from the desired signals, e.g., todistinguish the multiplexed signals from one another. The presentinvention reduces the effect of such interference by removing spectralcomponents of such interference in real-time. Specifically, the presentinvention digitizes the received signal collection and any interferenceinto quantized levels, and then Fourier transforms these digitizedsignals to obtain a plurality of spectral bins. Each frequency bin hasan intensity that corresponds to the composite power of any signal orinterference components that fall within the frequency bin. Some, if notall, of the spectral bins contain spectrally resolved portions of thedigitized group of signals. Additionally, at least one spectral bincontains the interference, and indeed multiple bins may containinterference, depending on the spectral content of the interference.

The present invention provides that any spectral bins that containinterference may be excised, if such bins contain a particularly highpower and if the collective power level of all of the bins, together,exceeds a predetermined threshold. Excising interference-containing binswhen both of such conditions are met may be useful because if thecollective power level of all of the bins is sufficiently low that thereceiver operates within its linear range, then any interference presentmay not necessarily cause additional distortion of the desired signals.In contrast, if the collective power of all of the bins is relativelyhigh, running the risk that the receiver may operate within itsnonlinear range, then such power level may be used as a signal thatinterference is present. Appropriate circuitry within the receiver mayidentify any interference present based on its power level, which may berelatively high as compared to the desired group of signals. Any bins soidentified as having interference may be excised, e.g., set to zero, andthe remaining bins then inverse Fourier transformed to obtain thedigitized signals—less at least some of the interference—in the timedomain. After high level interference is excised, the dynamic range ofthe remaining signal collection is reduced from its previous value whenhigh level interference was present. This reduced dynamic range afterinterference excision of high level interference advantageously reducesthe power consumption of the electronics following the interferenceexcision. The present invention thus may in some circumstances be viewedas a dynamic digital prefilter to maintain linear receiver operation.

First, an overview of the inventive system and components therein willbe provided. Then, methods of using such a system, and signals processedthereby, will be described. Lastly, some alternative embodiments andexemplary applications of the present invention will be described.

FIG. 1A illustrates a system 100 for reducing narrow bandwidthinterference with broad bandwidth signals in a receiver, according tosome embodiments of the present invention. Specifically, receiver 10 mayinclude an antenna/analog conditioner 11 configured to receive signals,one or more demodulators 12 configured to decode the informationcontained within the received signals, and interference reduction system100 disposed therebetween. In the illustrated embodiment, interferencereduction system 100 includes analog-to-digital (A/D) converter 110configured to receive and digitize signals received by antenna/analogconditioner 11, fast Fourier transform (FFT) circuit 120 configured toreceive and perform an FFT of the digitized signals output by A/Dconverter 110, power analysis circuit 130 configured to receive,analyze, and reduce interference contributions to the Fouriertransformed signals output by FFT circuit, and inverse FFT (iFFT)circuit 140 configured to transform the processed signals output bypower analysis circuit 130 back into the time domain. In the embodimentillustrated in FIG. 1A, iFFT circuit 140 may be configured to output theinverse-transformed signals to one or more demodulators 12 of receiver10.

Antenna/analog conditioner 11 illustrated in FIG. 1A may be configuredto wirelessly receive a group of signals within a particular frequencyband or bands. For example, antenna/analog conditioner 11 may beconfigured to receive analog CDMA or FDMA signals that fall within apre-defined spectral band, and may include one or more filtersconfigured to block signals having frequencies that fall outside of thisband. Appropriate antenna designs for a variety of signals in a varietyof contexts, e.g., terrestrial, aircraft, or space-based antennas, areknown in the art. In some embodiments, antenna/analog conditioner 11 maybe a pre-existing structure to which inventive system 100 may becoupled.

Antenna/analog conditioner 11 also may include an input RF filter toselect the bandwidth containing desired signal components and rejectother signals at frequencies outside of that bandwidth, a low noiseamplifier to establish the system noise level, and may contain one ormore downconverters to translate the RF bandwidth containing the usersignals into the bandwidth over which the A/D 110 operates. Suchcomponents may be considered together to constitute analog conditioningcircuitry.

A/D converter 110 may include input port 111 configured to be coupled toantenna/analog conditioner 11 via conductive element 112, such that A/Dconverter receives any analog signals received by antenna/analogconditioner 11. Conductive element 112 may include, for example, acoaxial cable, a transmission line, or any other suitable conductorconfigured to transmit signals within the pre-defined spectral band fromantenna/analog conditioner 11 to A/D converter 110 via input port 111.A/D converter 110 is configured to digitize and quantize the signalsthat it receives from antenna/analog conditioner 11. As known to thoseof skill in the art of digital signal processing, A/D converters arecommercially available devices that generate a digital version of ananalog signal by sampling that signal at a specified rate, and mappingthe power levels of that analog signal onto quantization levels in adigital data stream. A/D converters may have a fixed resolution, thatis, may have a fixed number of quantization levels onto which it may mapthe power levels of the analog signal. For example, A/D converters withan 8-bit resolution may be configured to map analog power levels onto255 quantization levels. Note that in some embodiments, antenna/analogconditioner 11 may include its own A/D converter configured to digitizethe received signals, or even may receive the desired group of signalsin a digital format, in which circumstances A/D converter 110 may beomitted from system 100, and antenna/analog conditioner 11 instead maybe directly coupled to FFT circuit 120. In embodiments including A/Dconverter 110, the converter provides as output to FFT circuit 120 viainput port 121 of FFT circuit 120 and conductive element 122 adigitized, quantized version of the desired group of signals, and adigitized, quantized version of any interference that shares the samespectral band as the desired group of signals (e.g., that was notfiltered out by antenna/analog conditioner 11 nor removed as a productof digitization or quantization).

As is known in the art, a discrete Fourier transform (DFT) may be usedto determine the frequency components of a signal that varies in time.An FFT is a particular variant of DFT, in which the input signal has anumber of points N that is a power of two, and is Fourier transformedusing an algorithm that is particularly efficient at obtaining a DFT ofan input signal having a number of points that is a power of two, suchas the Cooley-Tukey algorithms known in the art. The output of a DFT isa spectrally resolved version of the input signal, in which differentspectral components of the incoming signal are mapped onto a predefinednumber (e.g., for an FFT, a power of two) of spectral “bins.” In someembodiments, the signal output by A/D converter 110 has a number ofpoints N that is a power of two, and FFT circuit 120 is configured toperform an FFT on that signal. Circuits for performing FFTs, as well asother types of Fourier transformations of digital signals, are known inthe art and are commercially available. In some embodiments, FFT circuit120 provides as output a plurality of spectral bins, at least some ofwhich contain portions of the digitized group of signals, and at leastone of which contains interference. Each bin has a power levelcorresponding to the summed power levels of any spectral components—bethey based on the desired group of signals or based on theinterference—that have been mapped to that bin. FFT circuit 120 providessuch spectral bins to input port 131 of power analysis circuit 130 viaconductive element 132.

Preferably, antenna/analog conditioner 11, A/D converter 110, and FFTcircuitry 120 are configured so as to ensure that linear operation ismaintained prior to interference excision for the highest anticipatedinterference level. Preferably, the analog components (e.g.,antenna/analog conditioner 11) have a sufficiently high 1 dB compressionvalue relative to the input to maintain linear operation. In practice,the gain distribution may be examined and in some cases, the receivernoise temperature may be increased somewhat by reducing the analog gainvalues to achieve the required linearity. The clipping levels andquantization used in the digital technology (e.g., A/D converter 110 andFFT circuitry 120) likewise may be selected to avoid digital overflow ornonlinear operation in the digital technology used prior to interferenceexcision.

Power analysis circuit 130 is configured to excise one or more of thespectral bins that it receives from FFT circuit 120 based both on thecollective power of all of the bins, and based on the power of each binindividually. By “excise” it is meant that power analysis circuit 130reduces the power levels of such spectral bins to about zero in someembodiments, or to a predetermined non-zero level in other embodiments.Specifically, power analysis circuit 130 is configured to compare thecollective power of all of the bins to a predefined threshold, whichpreferably is based on the linear range of operation of one or moredemodulators 12. In one illustrative example, if the demodulator islimited to a maximum input signal level to achieve the requireddemodulation linearity, then the predefined threshold may be establishedto limit the signal power output from iFFT circuit 140 to demodulator(s)12 to somewhat less than the maximum demodulator input level, e.g., to95% or less, or 90% or less, or 85% of less, than the maximumdemodulator input level. If power analysis circuit 130 determines thatthe collective power level of all of the bins exceeds the predeterminedthreshold, then the circuit establishes a dynamically defined thresholdselected to facilitate identification and excision of interference thatmay be present, while reducing the risk that the desired group ofsignals also may be excised. To do so, power analysis circuit 130 maydetermine the amount by which the collective power level of all of thebins must be reduced to satisfy the predetermined threshold, and thenmay establish the dynamically defined threshold at such a value that, ifbins having powers exceeding that threshold are excised, it would reducethe collective power to or below the predetermined threshold. Poweranalysis circuit 130 then may excise any bins that exceed thedynamically defined threshold, e.g., by setting the power levels of suchbins to zero.

Power analysis circuit 130 may include any suitable circuitry configuredto store the predefined threshold, to compare the collective power ofthe spectral bins to the predefined threshold, to establish thedynamically defined threshold, and to excise any bins that exceed thedynamically defined threshold. For example, in the embodimentillustrated in FIG. 1A, power analysis circuit 130 may includearithmetic circuit A 133, storage medium 134, comparator A 135,arithmetic circuit B 136, and comparator B 137. Arithmetic circuit A 133may be configured to obtain the spectral bins output by FFT circuit 120,e.g., via conductive element 132 and any other suitable conductors, andto sum the power levels of the spectral bins so as to obtain acollective power level of the bins. Storage medium 134 may be configuredto store the predefined threshold. In some embodiments, the predefinedthreshold is based on the known linearity characteristics of circuitryin demodulator(s) 12, which may be established at the time receiver 10is designed and constructed. Comparator A 135 may be coupled, e.g., viaappropriate conductive elements, to storage medium 134, from which itreceives the predefined threshold, and to arithmetic circuit A 133, fromwhich it receives the spectral bins as well as the collective powerlevel of the spectral bins. Comparator A 135 may be configured tocompare the collective power of the spectral bins to the predefinedthreshold.

Comparator A 135 also may be coupled to arithmetic circuit B 136, e.g.,via an appropriate conductive element, so as to provide to arithmeticcircuit B 136 with the spectral bins, as well as a signal indicating theamount (if any) by which the collective power of the spectral binsexceeds the predefined threshold. Arithmetic circuit B 136 also mayreceive the predefined threshold, either from comparator A 135 or fromstorage medium 134, as is illustrated in FIG. 1A. Arithmetic circuit B136 is configured to obtain a dynamically defined threshold having avalue such that, if bins having powers exceeding that threshold areexcised, it would reduce the collective power to or below thepredetermined threshold. Arithmetic circuit B 136 is coupled tocomparator B 137, e.g., via an appropriate conductive element.Comparator B 137 receives the spectral bins, as well as the dynamicallydefined threshold from arithmetic circuit B 136. Comparator B 137 isconfigured to set to excise, e.g., set to zero, any spectral bins thatexceed the dynamically defined threshold.

Note that arithmetic circuits A and B 133, 136, comparators A and B 135,137, and storage medium 134 may be implemented using any suitable logiccircuits known in the art. For example, arithmetic circuits are known inthe art and are commercially available, as are comparators and storagemedia, and suitably may be coupled together with conductive elements. Inother embodiments, the functionalities of one or more of arithmeticcircuits A and B 133, 136, comparators A and B 135, 137, and/or storagemedium 134 may be provided by a suitably programmed field-programmablegate array (FPGA) or application-specific integrated circuit (ASIC).FPGAs and ASICs are commercially available, and methods of programmingsame to achieve desired logical programming are known in the art. Instill other embodiments, the functionalities of one or more ofarithmetic circuits A and B 133, 136, comparators A and B 135, 137, andstorage medium 134 may be provided by a suitably programmed computer,e.g., a personal computer. Additionally, note that circuitry other thanarithmetic circuits A and B 133, 136, comparators A and B 135, 137, andstorage medium 134 may be used to provide power analysis circuit 130with functionality analogous to that described herein.

Inverse FFT (iFFT) circuit 140 is configured to receive the spectralbins output by power analysis circuit 130 (e.g., by comparator B 137)via iFFT circuit input port 141 and conductive element 142. iFFT circuit140, which may include any suitable commercially available circuitry,then performs an inverse function to that of FFT circuit 120, that is,to determine the time components of a signal that varies in frequency,and as such to provide as output a signal that resembles the combinationof the desired group of signals plus interference initially received byantenna 11, but less the interference in any bin whose power was excisedby power analysis circuit 130, e.g., whose power was set to zero. iFFTcircuit 140 provides such output to demodulator(s) 12 via amplifierinput port 151 and conductive element 152. Because interferencereduction system 100 reduces the amount of interference that amplifierreceives, demodulator(s) 12 are more likely to operate within theirlinear range(s), and thus less likely to distort the desired group ofsignals. As such, receiver 10 may more readily distinguish, and obtaininformation from, the different multiplexed signal components of thedesired group of signals from one another than may otherwise be possiblewithout the interference reduction.

In an alternative embodiment, transponder 10′ illustrated in FIG. 1B isconfigured similarly to receiver 10 illustrated in FIG. 1A, but isconfigured to transmit the inversely Fourier transformed signals to aremote user for remote demodulation, rather than locally demodulatingthe signals. In transponder 10′, iFFT circuit 140 is configured toprovide its output to amplifier and transmit antenna(s) 13, e.g. viaamplifier input port 151 and conductive element 152. Amplifier andtransmit antenna(s) 13 then amplify and transmit the received signal toone or more individual user(s), each of whom has their owndemodulator(s) 12. In such embodiments, the predetermined threshold usedby power analysis circuit 130 is based on the linear range of operationof amplifier and transmit antenna(s) 13. For example, if the amplifierand transmit antenna(s) 13 are limited to a total power of 50 Watts orless to achieve linearity, then the predefined threshold may beestablished to limit the signal power output from iFFT circuit 140 toamplifier and transmit antenna(s) 13 to 50 Watts.

The operation of system 100 illustrated in FIGS. 1A-1B now will bedescribed in greater detail with reference to method 200 illustrated inFIG. 2 and exemplary signals illustrated in FIGS. 3A-4D. Method 200includes digitizing a group of signals and interference received from anantenna and analog conditioning circuitry of a receiver (step 210). Forexample, as described above with reference to FIG. 1A, A/D converter 110may receive and digitize signals from antenna/analog conditioner 11.Such signals may include the desired group of signals and anyinterference that occurs within the same bandwidth as does the group ofsignals. For example, as illustrated in FIG. 3A, exemplary code divisionmultiple access (CDMA) signal 301 received by antenna/analog conditioner11, sampled at a given period of time, and provided to A/D converter 110may include several different portions a-f having power levels that varyover time. In the example of FIG. 3A, signal 301 may include portionsa-d and f having a relatively low power level, as well as portion ehaving a higher power level. Portions a-d and f contain a desired groupof signals and perhaps low level interference components. The relativelysmall variation in their relative power levels may result from usersaccessing and leaving the system, and perhaps some variation in lowlevel interference, which typically persists a prolonged period of timein the normal course of system operation. However, signal 301 alsoincludes portion e which includes high level interference added to thedesired group of signals and perhaps some low level interference aswell.

As noted above, during step 210 of FIG. 2, A/D converter 110 digitizesthe received signals into quantized levels forming a digital datastream, which include the desired group of signals and interference.Following transforming the received signal collection into the digitaldomain, an FFT is performed (step 220) that transforms the receivedsignal collection at a given instant of time into frequency binsspanning the spectrum of the signal at that instant. In one illustrativeembodiment, received signal 301 has a bandwidth of approximately 10 MHz,and is mapped by FFT circuit 120 onto 64 bins, in which case thefrequency resolution (the width of each bin) is about 156 kHz.

In one illustrative example, if the instant of time when the FFT isperformed lies within the portion e in FIG. 3A, e.g., when stronginterference is present, a distribution of the power levels in thefrequency bins in FIG. 3B may be obtained. The actual signals in the FFTbins are contained in a digital data stream and the illustration in FIG.3B indicates the power levels in those bins at a given instant of time.If the input signal collection consisted exclusively CDMA signals, thepower levels in the FFT bins would be relatively constant. However, thedistribution of power levels in the frequency bins illustrated in FIG.3B contains two bins, B and D, that contain higher power levels than theremaining bins A, C, E, and F. These bins with the discernably higherpower levels contain not only the spectral components of the desiredCDMA signal collection but also the spectral components of twointerfering signals. The illustration in FIG. 3B also includes thedynamically predefined threshold level 322 established by the poweranalysis circuit 130 as described further below. In FIG. 3B, the powerlevel of bin B exceeds threshold 322, while the power level of bin Ddoes not.

Method 200 of FIG. 2 also includes obtaining the collective power of thespectral bins, and comparing the collective power level of those bins toa predefined threshold (step 230). For example, power analysis circuit130 described above with reference to FIG. 1A may receive the spectralbins from FFT circuit 120, and may include circuitry such as arithmeticcircuit A 133, storage medium 134, and comparator A 135 that togetherare configured to obtain the collective power of the received spectralbins, and to compare the collective power level of those bins to apredefined threshold so as to assess whether the desired group ofsignals and the interference together may have a power that exceeds thelinear range of demodulator(s) 12. For the exemplary signal illustratedin FIG. 3B, arithmetic circuit A 133 may obtain a sum of the powerlevels in bins A-F, storage medium 134 may store a value representativeof the maximum power level at which demodulator(s) 12 may demodulate thesignal(s) with adequate linearity, and comparator A 135 may beconfigured to compare the sum from arithmetic circuit A 133 to the valuefrom storage medium 134.

Method 200 of FIG. 2 also includes, if the collective power of thespectral bins exceeds the predetermined threshold, establishing adynamically defined threshold (step 240). Preferably, the dynamicallydefined threshold has a value that, if the bins having powers exceedingthat threshold are excised, the collective power would be reduced to orbelow the predetermined threshold. For example, as noted above withreference to FIG. 1A, power analysis circuit 130 may include arithmeticcircuit B 136 that is configured to establish the dynamically definedthreshold in such a manner. For the exemplary spectral bins illustratedin FIG. 3B, bin B has a power level that exceeds dynamically definedthreshold 322, while the power level of bin D is less than threshold322.

Method 200 of FIG. 2 also includes excising any bins that exceed thedynamically defined threshold (step 250), such as by setting the powerlevel of such bins to zero. For example, power analysis circuit 130illustrated in FIG. 1A may include comparator B 137 configured tocompare the power level of each spectral bin to the dynamically definedthreshold, and to reduced to zero the power level of any spectral binthat exceeds the dynamically defined threshold. Referring to FIG. 3C, itmay be seen that the power level of spectral bin B which exceededdynamically defined threshold 322 in FIG. 3B has been set to zero, thusexcising that bin and resulting in modified bin B′. However, althoughthe power level of spectral bin D exceeds that of the remaining bins,bin D in FIG. 3B has not been excised because its power does not exceeddynamically defined threshold 322. The signals at this time instant arecontained in a digital data stream that differs from the digital datastream after the A/D 110 by excluding the spectral distribution of thehigh level interference in frequency bin B′ but containing the spectraldistribution of the interference below the threshold in frequency bin D.The power levels after excision in FIG. 3C represent the spectralcomponents in the data stream after excision. It should be noted thatbin D may contain usable signal information, because the power level ofthe interference is not significantly greater than that of the desiredsignal component to which it is added. In this case, the CDMA processinggain mitigates the lower level interference as is known in the art.Preferably, following the excision of any bins that exceed thedynamically defined threshold, the collective power of the remainingbins is at or below the predetermined threshold, that is, is within thelinear range of operation of demodulator(s) 12.

Method 200 of FIG. 2 further includes performing an inverse Fouriertransform on the remaining spectral bins, so as to obtain a digitizedgroup of signals less any signal contributions and interference that wasin the excised bins (step 260). For example, as noted above withreference to FIG. 1A, iFFT circuit 140 may perform an iFFT on the outputof power analysis circuit 130 (e.g., on the output of comparator B 137),and may provide the output of such iFFT to demodulator(s) 12. The iFFToutput includes a digital data stream containing the informationdescribing the wide bandwidth CDMA spectrum illustrated in FIG. 3D at agiven instant of time occurring during portion e illustrated in FIG. 1A.The resulting spectrum after iFFT operations is again a digital datastream that at a given instant of time may have the power spectraldistribution illustrated in FIG. 3D during the instant of time theseoperations were performed within the time interval e in FIG. 3A. Eachfrequency bin after iFFT operations may follow the time domain signalvariation during subsequent time periods.

Note that the spectrum after excision in FIG. 3D has reduced desiredCDMA signal power that results from the excision. This signal loss issmall as the high level interference power has a narrow bandwidth thatoccupies a small fraction of the frequency bins. Wide bandwidth highlevel interference occupies a larger number of frequency bins thatresults in more desired CDMA signal power loss. The tolerable amount ofsignal loss depends on the link margin and amount of signal energyneeded for acceptable signal performance. However, excising high levelsignal components that result in nonlinear receiver operation isrequired to avoid the additional degradation to communicationperformance. When linear receiver operation is maintained, the loss incommunication performance is limited to desired signal loss resultingfrom excision. In addition, the spectral components of the interferencewhose levels do not exceed the excision threshold are also present. Theprocessing gain of the CDMA waveform inherently provides interferenceprotection from low level interference. These issues are known in theart.

FIG. 3E illustrates exemplary CDMA signal 301′ following digitization,excision, and inverse Fourier transformation of signal 301. As comparedto signal 301, it may be seen that each portion a-e of signal 301′ has arelatively low power level. The relatively small variation in theirrelative power levels may result from users accessing and leaving thesystem, and perhaps some variation in low level interference, whichtypically persists a prolonged period of time in the normal course ofsystem operation. Portion e′, which in signal 301 included high levelinterference added to the desired group of signals and perhaps some lowlevel interference as well, has now been reduced to a power levelsimilar to that of other portions a-d and f of signal 301′.

Following steps 210-260 illustrated in FIG. 2, the resulting signalsthen may be distinguished from one another, e.g., demultiplexed anddecoded using a priori knowledge of the CDMA codes initially used tomultiplex the signals with one another. Receiver 10's power requirementsfor performing such processing may be significantly reduced relative tothose for processing otherwise similar signals from which interferencehad not been excised particularly in the case where interferenceexcision is applied to transponder architectures where the transponder'stransmitter level must be increased to maintain a linear output.

Note that excising a given bin during step 250 of method 200 illustratedin FIG. 2 not only removes any interference within that bin, but alsoany spectral components of the desired group of signals within that bin.However, for CDMA-based signals, such excision of desired signalcomponents may have little impact on the receiver's ability later todemultiplex and decode the signal components in the remaining bins.Specifically, as mentioned above and as known in the art, CDMA spreadseach signal of the group over the same selected portion of the spectrumas the other signals in the group. As such, excising a subset of thatselected portion of the spectrum (the subset being within the excisedbin) reduces the overall signal strength of all of the signals in thegroup, but substantially without reducing the information content of thesignals in the group. For example, even if 20% of the selected portionof the selected portion of the spectrum is excised, the overall power ofthe desired group of signals may be reduced by approximately 1 dB, whichmay not significantly impact the receiver's ability to demultiplex anddecode the signal components in the remaining bins. However, if theexcised interference has sufficiently large bandwidth relative to thatof the desired group of signals, then the overall power of the desiredgroup of signals may be reduced to an extent that may make it difficultto demultiplex and decode the signal components in the remaining binsthus degrading communication performance. Thus, the interferencepreferably has a relatively narrow bandwidth relative to the desiredgroup of signals.

Additionally, note that CDMA is only one example of a technique that maybe used to multiplex a group of signals with one another in a “broadbandwidth” manner, from which interference may be excised using system100 illustrated in FIGS. 1A-1B and method 200 illustrated in FIG. 2.Another such technique is FDMA, which as mentioned above assignsdifferent signals in the group to different portions of the spectrumthan one another. FIGS. 4A-4D illustrate exemplary signals that may begenerated using system 100 illustrated in FIGS. 1A-1B and method 200illustrated in FIG. 2 to excise interference from an FDMA signal.

Specifically, FIG. 4A illustrates exemplary FDMA signal 401 which may bereceived by antenna 11 and provided to A/D converter 110 as illustratedin FIG. 1A (step 210 of method 200 illustrated in FIG. 2). FDMA signal401 may include several different portions a-i that vary relative to oneanother over time and differ from one another, depending on theparticular power level for each signal within the frequency bins Signal401 also may include interference, as denoted by higher power levelportion f.

FIG. 4B illustrates an exemplary distribution of power levels infrequency bins following digitization and Fourier transformation ofsignal 401 (step 220 of method 200 illustrated in FIG. 2) at an instantduring portion f illustrated in FIG. 4A. The output of Fouriertransformation of FDMA signal 401 includes a digital stream having aplurality of spectral bins A-N to which different spectral components ofsignal 401 may be mapped by FFT circuit 120 during step 220 of FIG. 2.As may be seen in FIG. 4B, the power levels in each bin corresponding tothe portions of the digital data stream containing the spectralinformation of that bin's content, the power levels in the variousspectral bins may vary relative to one another, with spectral bins J andM having particularly high power levels relative to the others, and thuspossibly containing interference. The collective power of the spectralbins then may be obtained and compared to predefined threshold, e.g., byimplementing step 230 illustrated in FIG. 2 using arithmetic circuit A133, storage medium 134, and comparator A 135 described above withreference to FIG. 1A. If the collective power of the spectral binsexceeds the predetermined threshold, then dynamically defined threshold423 may be established, as described above with reference to step 240 ofFIG. 2 and arithmetic circuit B 136 described above with reference toFIG. 1A. For the exemplary spectral bins illustrated in FIG. 4B, onlybin M has a power level that exceeds dynamically defined threshold 423.FIG. 4C illustrates the spectral bins following excision of bin M in amanner analogous to that described above with reference to step 250 ofFIG. 2 as implemented using comparator B 137 described above withreference to FIG. 1A, resulting in modified bin M′. In this regard, itshould be noted that bin J may contain usable signal information,because the power level of the interference therein is not significantlygreater than that of the desired signal components to which it is added.

In addition, some of the frequency bins may be unpopulated with signalcomponents and be occupied by only noise components that have nocommunication value. The example in FIG. 4B contains desired FDMA signalcomponents in bins B, D, F, G, I, K, L, and N, and may or may not havedesired signal components in bins J and M that are masked byinterference. Bins A, C, E, and H illustrated in FIG. 4B contain noisecomponents but substantially no desired FDMA signal components, and thusare denoted as empty bins. The distribution of signal components isillustrated for a given instant of time and varies with operation.

FDMA-based systems may provide communications to a number of usersthrough a transponder architecture and the illustration in FIG. 4Bdepicts the distribution within the transponder's IF (intermediatefrequency) bandwidth. Individual users may receive the entire signalcollection communicated by the transponder and select the pre-assignedfrequency slot(s) to receive the communications intended for their ownuse. As discussed above, the transponder's transmitter preferablyremains linear, and if high level interference is not excised, thetransmitter power output may be increased to remain linear for thehighest anticipated interference level, thus incurring a significantincrease in power consumption. In cases where interference protection isdesired, frequency hopped spread spectrum techniques may be used wherethe carrier frequency is hopped in a pseudo random pattern known to thetransponder's transmitter and system users but not to the interferencesource. In such cases, the interference source may dilute its resourcesby distributing the interference over the wider spread spectrumbandwidth or cover a limited portion of that bandwidth and interferewith user communications only part of the time. The distribution in thefrequency bins illustrated in FIG. 4B again pertains for an instant oftime and preferably covers the hop bandwidth and interference at othertimes may be outside the hop bandwidth, because the pseudo-randomhopping sequence is unknown to the interference source. The degradationto system users when the interference coincides with the hoppedbandwidth may be mitigated by error correction coding and interleavingas is known in the art.

As described further above with reference to FIGS. 1A-1B and 2, poweranalysis circuitry 130 may obtain the collective power of the spectralbins, compare that power level to a predefined threshold, and, if thecollective power exceeds that predefined threshold, dynamicallyestablishe an interference threshold to determine potential frequencybins having sufficient power to result in nonlinear operation (steps230-250). Thus in the illustrative example in FIG. 4B, frequency bin Mcontaining strong interference, e.g., having a power level greater thandynamically defined threshold 423, would be excised, while frequency binJ having a power level less than the dynamically defined threshold 423would not be excised. As in FIG. 3B, the spectral information in thesignal collection illustrated in FIG. 4B is a digital bit stream, andthe power levels in the frequency bins is used for illustrationpurposes.

Optionally, additional benefits may be obtained by excising certainspectral bins in addition to those that exceed dynamically definedthreshold 423. For example, turning back to FIG. 4B, it may be seen thatthe power levels of bins B, D, F, G, and I-N are greater than empty binthreshold 421, which designates a power level below which a bin may beconsidered to carry insufficient information to distinguish, and thus is“empty”. Empty bin threshold 421 may be defined, for example, based on apriori knowledge about the system noise level and the variance of itsvalue. That is, each of bins B, D, F, G, and I-N has sufficient power asto permit communications (noting, of course, that bin M also includeshigh-power interference). By comparison, bins A, C, E, and H have powerlevels that are below empty bin threshold 421, and may be excised bysetting their values to zero, such as illustrated in FIG. 4C, resultingin modified bins A′, C′, E′, and H′. After excision of the high levelinterference and empty bins that have no communication value, theoverall quantization level of the desired group of signals may bereduced allowing communication of the useful information contentthereof.

Thus, method 200 illustrated in FIG. 2 optionally includes steps ofcomparing the power levels of the spectral bins to an empty binthreshold and excising any bins having power levels that are lower thanthat threshold. Such steps may be executed, e.g., using a comparator tocompare the bins' power levels to a threshold stored in a suitablestorage medium and to excise any bins having power levels that are lowerthan that threshold. Such comparator and storage medium may be includedin system 100 illustrated in FIG. 1, e.g., by providing dedicatedcircuitry configured to provide the above-described functionality, or byproviding a suitably programmed FPGA, ASIC, or computer, such asdescribed above with reference to FIGS. 1A-1B.

FIG. 4B also depicts another threshold level for desired user signalpower. FDMA systems communicate the signals of multiple users that arerouted to the transponder's transmitter. This transmitter has a fixedoutput power so that the transmitted signal level is desired to bemaintained in its linear operating range. If a user or group of usersraise their power levels, the resulting transmitter power may exceed thelinear operating range of the transmitter, causing the communicationperformance to all users to degrade. Embodiments of the presentinvention may be used to implement user power control to limit userpower levels so that all users have an equable portion of thetransmitter's power and are not degraded by nonlinear transmitteroperation. Specifically, the distribution of user power levels may beobtained based on the power levels in the frequency bins, and thus thedigital circuitry used in interference excision advantageously providesa means to monitor the effectiveness of user power control techniques.In the example illustrated in FIG. 4B, users' signal components infrequency bins B, F, I, J, L, and M exceed the desired user power level422 (noting that bins J and M also contain interference components,which may obscure the users' actual signal levels). Additionally, as maybe seen in FIG. 4B, users' signal components in frequency bins A, C-E,H, and K have less than the desired user power level 422, and thus havepotentially degraded communication performance (noting that bins A, C,E, and H otherwise may be designated as “empty” as described above).Additionally, users' signal components in frequency bins G and N haveapproximately the desired user power level 422. The desired user powerlevel 422 for user power control monitoring may be established by apower analysis circuit similar to 130, and which may be used after otherfrequency bins are excised, e.g., after bins having high interferencelevels and/or empty bins are excised. In some embodiments, the desireduser power level 422 may be established dynamically by summing the powerlevels in frequency bins that have not been excised so that thetransponder's transmitter remains within its linear operating range.After user power deviations from the desired levels are determined,users can be notified to readjust their power levels. For example, binshaving power levels that are lower than desired user power level 422 maybe identified, and sources of the signal components within thoseidentified bins may be notified that they may increase their powerlevels so as to improve performance.

Following excision of any spectral bins having power levels that exceeddynamically defined threshold 423, and optionally of any spectral binshaving power levels that are less than empty bin threshold 421, as wellas optionally identifying bins having power levels that are higher orlower than desired user power level 422, an inverse Fourier transformmay be performed on the remaining spectral bins so as to obtain adigitized group of signals less any signal contributions andinterference that was in the excised bins (step 260 of FIG. 2). An iFFTcircuit 140 such as described above with reference to FIG. 1A may beemployed to perform such an inverse Fourier transformation and mayprovide the output of such transformation to demodulator(s) 12. However,the power levels of interference portion M′ and the empty bins A′, C′,E′, and H′ illustrated in FIG. 4C, which have no communication value,have been reduced significantly relative to their power levels in FIG.4B. The overall quantization level of the remaining group of signals,including “empty” bins and remaining interference, is significantlylower than in FIG. 4B, and preferably is within the linear range ofdemodulator 12. As such, system 100 may process the desired group ofsignals and any remaining interference, which may be of a sufficientlylow level as to inhibit distortion of the desired group of signalsduring amplification.

FIG. 4D illustrates exemplary FDMA signal 401′ following digitization,excision, and inverse Fourier transformation of signal 401. As comparedto signal 401, it may be seen that each portion a-i of signal 401′ has arelatively low power level. Portion f′, which in signal 401 includedhigh level interference, has now been reduced to a power level similarto that of other portions a-e and g-i of signal 401′.

The signals of the group then may be distinguished from one another,e.g., demultiplexed using a priori knowledge of the spectral regions towhich each of the signals has been assigned. Receiver 10's powerrequirements for performing such processing may be significantly reducedrelative to those for processing otherwise similar signals from whichinterference had not been excised particularly in transponderarchitectures.

As compared to the CDMA example described above with reference to FIGS.3A-3C, in which excising bins resulted in an overall reduction in CDMAsignal strength but without loss of information, it should be noted thatexcising interference-containing bins in FDMA-based signals also mayexcise any desired signals assigned to spectral regions that fall withinthe excised bins. However, in such bins, the interference may besufficiently strong that even without excision it may not necessarily bepossible to obtain information from the desired signals, and as such theexcision may not necessarily reduce the amount of information thatpracticably may be obtained from the group of signals. The tolerableinterference level depends on the error correction coding andinterleaving commonly practiced in the art.

Note that the operation of interference reduction system 100 illustratedin FIGS. 1A-1E, e.g., the implementation of method 200 illustrated inFIG. 2 to produce signals as exemplified in FIGS. 3A-3E and 4A-4D,preferably occurs in “real-time.” That is, the components of system 100preferably reduce interference with broad bandwidth signals as thosesignals arrive based on the relative power levels of the differentspectral components of the signal, and immediately thereafter providethe resulting signals to demodulator(s) 12 for processing, asillustrated in FIG. 1A, or to transmit antenna(s) 13 for transmission toa uwer, as illustrated in FIG. 1B. By comparison, adaptive notch filterssuch as mentioned above rely on iterative spectral analysis of theinterference to derive the adaptive filter's weighting coefficients,potentially resulting in time delays in adjusting to changes when theinterference spectrum dynamically varies over time in an unpredictablemanner.

Note that for FDMA-based signals, system 100 may be adapted to provideenhanced functionality in routing signals to various destinations, e.g.,analogously to a digital channelizer. For example, communicationsatellites and terrestrial transponders may use multiple beams (signals)to increase link performance and communication throughput incommunicating to an overall coverage area. In such designs, frequencyreuse plans wherein adjacent antenna beams are assigned a subband of theoverall frequency allocation and groups of FDMA users populate thosesubbands may be used to reduce mutual interference between adjacentbeams. The investment in digital technology in system 100 together withdigital routers and multiplexers advantageously provides operationalflexibility to provide connectivity between users distributed over thecollection of antenna beams accessing system 100 units connected to eachreceiving beam to their destination beams that may or may not be in beamlocations where the signal originates. However, the demands forcommunication throughput often are not uniformly distributed over thecollection of beams, and users at one beam destination may desirecommunication to other beam destination(s).

FIG. 5 illustrates alternative transponder 50 configured to use modifiedsystem 500 to digitally demultiplex, excise interference from,multiplex, and/or route signals to multiple users. Specifically,transponder 50 illustrated in FIG. 5 receives analog conditioned signalsfrom one of the multiple antenna/analog conditioners 51 and transformsthese analog inputs to the digital domain using an A/D converter 510,preferably with M bits of quantization to ensure the analog input is notclipped and remains linear for the highest anticipated interferencelevel. The digital stream from the A/D converter then is digitallydemultiplexed using FFT circuit 520. That is, signals that may overlaptemporally with one another may be demultiplexed from one another byassigning their spectral contributions to different bins than oneanother. Power analysis circuit 530 then may excise any high power levelinterference and/or empty subbands in the demultiplexed signals, thusreducing the quantization requirements to a smaller number of bits(M−N). Then, iFFT circuit/signal router/multiplexer 540 receives thedemultiplexed frequency bins, less any excised bins, and routes the binsbased on signal components contained therein their destination outputs,e.g., to one of digital-to-analog (D/A) converters 52 and amplifier andtransmit antennas 53. Preferably, iFFT circuit/signal router/multiplexer540 further is configured to multiplex together, as appropriate,frequency bins received by one or more other antennas/analogconditioners 51 and then inversely Fourier transforms the multiplexedsignals from the spectral to the time domain. In some embodiments, iFFTcircuit/signal router/multiplexer 540 includes circuitry including anetwork of switches configured to receive the frequency bins output bypower analysis circuit 530 and to route those bins according to theintended destination of those bins. The circuitry of iFFT circuit/signalrouter/multiplexer 540 also may include a multiplexer circuit configuredto receive the routed bins from the network of switches and to multiplexthose bins with any other bins having the same intended destination. Thecircuitry of iFFT circuit/signal router/multiplexer 540 also may includean iFFT circuit configured to receive the multiplexed bins from themultiplexer circuit and to perform an iFFT to transform thosemultiplexed bins into the time domain. The resulting digital datastream, including the multiplexed outputs as appropriate, are thentransformed by D/A converter 52 into the analog domain, analogconditioned using frequency upconverters to the transmitted frequencyused in that beam position (circuitry not shown), and then amplified bythe transmitter and routed to the antenna serving the intendeddestination using an appropriate amplifier and transmit antenna 53. Notethat the circuitry of iFFT circuit/signal router/multiplexer 540 may beimplemented, for example, using an ASIC or FPGA, or by separatelyproviding and connecting the individual circuitry components togetherwith one another.

In other embodiments, system 100 usefully may be employed in digitalchannelizer 60 such as illustrated in FIG. 6 so as to provide enhancedcommunication flexibility. Digital channelizer 60 may include receiveantenna/analog conditioner 61 configured to receive a group ofmultiplexed signals, e.g., FDMA-based signals, as well as interferencereduction system 600 configured to excise interference and “empty” binstherein using method 200 described above with reference to FIG. 2 andoptional modifications thereof, e.g., A/D converter 610, FFT circuit620, power analysis circuit 630, and iFFT circuit 640 which may besimilar to those described above with reference to FIG. 1A. Analogouslyas noted above with reference to FIG. 5, FFT circuit 620 is configuredto demultiplex the signals of the group from one another, e.g., based onthe different sub-portions of the spectral region to which the signalsrespectively have been assigned. Digital channelizer 60 further includesone or more routers 65 that are configured to route the signals to oneor more multiplexer(s) 66 based on the intended destinations of thedifferent signals. That is, router(s) 65 group together signals that areintended for a common destination as one another, and provide thosesignals as a group to a corresponding multiplexer 66, which multiplexesthe signals of the group together and provides the resulting group ofsignals to a corresponding D/A converter 62 and then to a correspondingamplifier and transmit antenna 63. In some embodiments, digitalchannelizer includes as many router(s) 65, multiplexer(s) 66, D/Aconverters 62, and amplifiers and transmit antennas 63 as there areintended signal destinations.

In some cases, signal components after interference excision fromadjacent beam positions can advantageously be combined to produce anadditional beam position located within the coverage of the adjacentbeams, e.g., digital beamforming to provide the capability to increaseantenna gain levels within the coverage area compared to the fixed beampositions. Such digital beamforming can be implemented by using a prioriselected fixed beam combining coefficients, dynamically determined beamcombining coefficients to track system uses, or dynamically determinedcoefficients to adaptively produce pattern nulls in the direction ofresidual interference sources as known in the art. Because high levelinterference sources may be excised using system 100, these digitalbeamforming techniques may be implemented without degradation fromnonlinear receiver operation.

In other embodiments, the signals received by multiple beams may bedemodulated and remodulated into a transmitted fog mat in a processingtransponder architecture. In these embodiments, the demodulatorcircuitry may be configured to simultaneously demodulate groups ofindividual FDMA signal components. The capabilities of such demodulatorsare not fully utilized for individual beams that are lightly populatedwith FDMA user signals, In such cases, multiplexing individual FDMAsignals received from different antenna elements would be advantageousby utilizing the demodulators more effectively. This approach may alsoreduce the cost and complexity of the system by reducing the number ofdemodulators required. Thus, system 100 usefully may be employed toenhance transmission efficiency by facilitating combination of multiplelightly-populated groups of signals with one another prior todemodulation, remodulation, and transmission.

For example, FIG. 7A illustrates digital combiner 70 for FDMA signalsthat may include analog conditioned antenna inputs 71 configured toreceive signals from one or more receive antenna(s) (not illustrated)respectively configured to receive groups of signals from differentdestinations. Digital combiner 70 also includes A/D converter 710, FFTcircuit 720, and power analysis circuit 730 (which may be the same asdescribed above with reference to FIG. 1A), which may be configured toexcise empty bins and interference and to demultiplex signals in themanner described above. Digital combiner 70 also includes arouter/multiplexer/iFFT circuit 740 configured to select individualfrequency bins from the collection of demultiplexed antenna outputs,multiplex then into a signal data stream, and process them with an iFFTto form a single digital input data stream that is provided todemodulator/remodulator 750. After remodulation, the remodulated datastream in its converted modulation format can be demultiplexed intoseparated signal components and routed by demultiplexer/router 760 todestination antennas 770 for transmissions. The remodulated data streamsignals at each transmitting antenna 770 are processed by a D/Aconverter, analog conditioned into the transmitted frequency, amplifiedby the transmitter, and radiated by the antenna. In operation, theindividual signal components in the received antenna signals may beassigned to different non-conflicting frequency slots by a resourceallocator to avoid mutual interference. The assignments of the frequencyslots illustrated in FIG. 7B provides an illustrative example for threebeams i, j, and k where five frequency slots distributed among the threebeams are multiplexed into a single data stream without conflicting withone another. Additionally, embodiments of system 100 and method 200described above suitably may be modified to provide adaptivetransmission bandwidth. Previously known system designs are configuredto uniformly assign fixed bandwidth subbands to individual beampositions as illustrated in FIG. 8A Specifically, the spectralsub-regions to which different signals of a group are assigned typicallyis fixed based on the overall bandwidth available for the group ofsignals, e.g., each signal may be assigned to a sub-region of fixedwidth, regardless of the throughput requirements of the signal. Insituation where individual signals may have varying needs forcommunication throughput, it may be desirable to provide greaterbandwidth to signals having heavy demands for throughput, and lessbandwidth to signals with lesser demands for throughput. The bandwidthin this case may be established by the bandwidth of the multiplexedsignal components. For example, transponder 50 illustrated in FIG. 5,digital channelizer 60 illustrated in FIG. 6, and digital combiner 70illustrated in FIG. 7A suitably may be configured to assign differentsignals to spectral sub-portions having widths that vary relative to oneanother based on the throughput requirements of the respective signalsas illustrated in FIG. 8B. Such a configuration may provide enhancedoperational flexibility to vary the transmission bandwidth in accordancewith the time varying throughput demands of the individual signals.

EXAMPLE

Potential performance characteristics of system 100 were characterizedby measuring a CDMA signal having interference, excising thatinterference, and determining the bit error rate (BER) of the signalbefore and after the interference was excised, as well as for acomparable signal having no interference. Specifically, FIG. 9Aillustrates a measured wideband-CDMA signal centered at 0 MHz and havinga relative amplitude of approximately 15 dB between about −2.5 MHz and+2.5 MHz and a noise level with a relative amplitude of 5 dB. Thisspectral plot was obtained by transforming the digital data stream witha D/A converter to obtain the analog spectral distribution. Dividingthis spectral distribution into frequency bins and obtaining the powerin each bin would result in the spectral plots analogous to theexemplary plots in FIGS. 3B and 4B. Interference in the form of twocomponents each having a bandwidth of 25 kHz and an spectral powerdensity approximately 30 dB higher in the 25 KHz bandwidth was added tothe CDMA signal at spectral locations of about −0.5 MHz and +1.0 MHz.FIG. 9B illustrates a second measured signal spectrum again obtained byprocessing the measured digital data stream with a D/A converter toobtain the analog spectral plot that included the CDMA signal after theinterference was excised by setting the signal amplitude to zero inregions that previously contained the interference. A third signal (notillustrated) was simulated that was analogous to the CDMA signalillustrated in FIG. 9A but lacked interference. Each of the three typesof signals were measured for different values of energy per chip/noisespectral density (Ec/No) to obtain bit error rate and block error ratevalues to evaluate the effectiveness of excising the two interferingtone interferers.

The bit error rate (BER) of the three types of simulated signals for thedifferent values of energy per chip/noise spectral density (Ec/No) isillustrated in FIG. 10. As is known in the art, BER is reflective of theproportion of incorrect bits in a signal; a BER of 10⁻³ means that onebit in a thousand is incorrect. It may be seen that the CDMA signalwithout interference (closed squares) had the lowest BER at a givenEc/No, but that the CDMA signal with interference excised (closeddiamonds) had a BER that was not significantly higher than that of theCDMA signal without interference. By comparison, the CDMA signal withinterference present had a significantly higher BER than either of theother two signals that lacked interference, having a BER of 0.1 at anEc/No of about −20 dB, while the other two signals had a BER of 0.1 atan Ec/No of about −22 dB. Thus, the excision of the two interferingtones closely restored the system performance to the performance withoutinterference. In addition to these measurements, simulation resultsyielded the same conclusion.

The measurements and simulations used interference values that werewithin the linear operating range of the measurement instrumentation sothat the additional impacts of nonlinear operation were not observed.The CDMA signal used in these measurements contained a spreading code tobroaden its response but did not use error correction coding to reducethe interference effects as observed by the BER measurements. Normallysuch coding would have restored the BER performance and the effect ofthe interference would not be noticeable. The present invention hasemphasized the need to excise high level interference to maintain linearsystem operation and described a method to determine an interferencethreshold for that purpose. In some CDMA applications where theprocessing gain of the CDMA waveform is not adequate to restore the BERperformance without interference, reducing the interference thresholdlevel to excise additional interference components may be advantageous.

Thus, it may be seen that excising interference in the manner describedherein, e.g., using system 100 illustrated in FIGS. 1A-1B to implementmethod 200 illustrated in FIG. 2, may substantially reduce the BER ofbroad bandwidth, multiplexed signals, and thus may facilitatetransmission, reception, demultiplexing, and decoding of such signals.

While various illustrative embodiments of the invention are describedabove, it will be apparent to one skilled in the art that variouschanges and modifications may be made therein without departing from theinvention. For example, interference reduction system 100 may beconfigured to work with, and to be coupled to, a pre-existing receiver10 or transponder 10′, but need not necessarily be considered to be anintegral part of such a receiver or transponder, and indeed suitably maybe used with any circuitry that would benefit from interferencereduction. The appended claims are intended to cover all such changesand modifications that fall within the true spirit and scope of theinvention.

1. A system for processing a group of signals and interference, thesystem comprising: (a) an analog-to-digital (A/D) converter configuredto digitize the group of signals and the interference; (b) a Fouriertransform circuit coupled to the A/D converter and configured to obtaina Fourier transform of the digitized group of signals and theinterference and to provide as output spectral bins, each bin having apower level, at least some of the bins respectively containing portionsof the digitized group of signals, and at least one of the binscontaining the interference; (c) a power analysis circuit configured tocompare the collective power level of the spectral bins output by theFourier transform circuit to a fixed, predetermined threshold, and ifthe collective power level exceeds the fixed, predetermined threshold,to excise at least one bin that contains the interference such that thecollective power level following excision does not exceed the fixed,predetermined threshold and is sufficiently low to maintain linearoperation of the system; and (d) an inverse Fourier transform circuitconfigured to obtain an inverse Fourier transform of the remainingspectral bins and to provide as output a digitized group of signals lessthe interference in any excised spectral bin.
 2. The system of claim 1,wherein the power analysis circuit further is configured to obtain adynamically defined threshold having a value that, if spectral binshaving power levels exceeding that threshold are excised, would reducethe collective power level to or below the fixed, predeterminedthreshold, and wherein the excised spectral bin exceeds the dynamicallydefined threshold.
 3. (canceled)
 4. The system of claim 2, wherein thepower analysis circuit comprises a first arithmetic circuit configuredto obtain the collective power level, a storage medium configured tostore the fixed, predetermined threshold, and a first comparatorconfigured to compare the collective power level to the stored fixed,predetermined threshold.
 5. The system of claim 4, wherein the poweranalysis circuit further comprises a second arithmetic circuitconfigured to obtain the dynamically defined threshold, and a secondcomparator configured to compare the power level of each bin to thedynamically defined threshold and to excise any bin whose power levelexceeds that threshold.
 6. The system of claim 4, wherein the poweranalysis circuit comprises a field programmable gate array (FPGA) orapplication specific integrated circuit (ASIC) suitably programmed so asto include the first and second arithmetic circuits, the storage medium,and the first and second comparators.
 7. The system of claim 2, whereinthe power analysis circuit is configured to excise spectral bins havingpower levels that exceed the dynamically defined threshold by settingthe power levels of those bins to zero.
 8. The system of claim 1,wherein the power analysis circuit further is configured to compare thepower level of each spectral bin to an empty bin threshold, and if thepower level of a bin is less than the empty bin threshold, excising thatbin.
 9. The system of claim 1, wherein the group of signals comprisecode-division multiple access (CDMA) or frequency-division multipleaccess (FDMA)-based signals.
 10. The system of claim 1, furthercomprising an amplifier configured to amplify the digitized group ofsignals output by the inverse Fourier transform circuit and an antennaconfigured to transmit the amplified, digitized group of signals outputby the amplifier.
 11. The system of claim 1, further comprising ademultiplexer configured to demultiplex the digitized group of signalsoutput by the inverse Fourier transform circuit, at least one antenna,and at least one router configured to route the demultiplexed signals tothe at least one antenna based on the intended destination thereof. 12.The system of claim 1, wherein the A/D converter is configured to remainin its linear range for the highest anticipated interference level. 13.A method of processing a received group of signals and interference in asystem, the method comprising: (a) digitizing the received group ofsignals and the interference; (b) obtaining a Fourier transform of thedigitized group of signals and the interference to output spectral bins,each bin having a power level, at least some of the bins respectivelycontaining portions of the digitized group of signals, and at least oneof the bins containing the interference; (c) comparing the collectivepower level of the spectral bins to a fixed, predetermined threshold,and if the collective power level exceeds the fixed, predeterminedthreshold, excising at least one bin that contains the interference suchthat the collective power level following excision does not exceed thefixed, predetermined threshold and is sufficiently low to maintainlinear operation of the system; and (d) obtaining an inverse Fouriertransform of the remaining spectral bins and providing as output adigitized group of signals less the interference in any excised spectralbin.
 14. The method of claim 13, further comprising obtaining adynamically defined threshold having a value that, if spectral binshaving power levels exceeding that threshold are excised, would reducethe collective power level to or below the fixed, predeterminedthreshold, and wherein the excised spectral bin exceeds the dynamicallydefined threshold.
 15. (canceled)
 16. The method of claim 13, furthercomprising programming a field programmable gate array (FPGA) orapplication-specific integrated circuit (ASIC) to execute step (c). 17.The method of claim 13, comprising excising the spectral bins havingpower levels that exceed the dynamically defined threshold by settingthe power levels of those bins to zero.
 18. The method of claim 13,further comprising comparing the power level of each spectral bin to anempty bin threshold, and if the power level of a bin is less than theempty bin threshold, excising that bin.
 19. The method of claim 13,wherein the group of signals comprise code-division multiple access(CDMA) or frequency-division multiple access (FDMA)-based signals. 20.The method of claim 13, further comprising amplifying the digitizedgroup of signals output by the inverse Fourier transform circuit andtransmitting with an antenna the amplified, digitized group of signals.21. The method of claim 13, further comprising demultiplexing thedigitized group of signals output by the inverse Fourier transformcircuit and routing the demultiplexed signals to at least one antennabased on the intended destination thereof.
 22. The method of claim 13,comprising executing step (a) using an A/D converter configured toremain linear for the highest anticipated interference level.
 23. Themethod of claim 13, further comprising providing to an amplifier thedigitized group of signals output by the inverse Fourier transformcircuit, wherein the fixed, predetermined threshold is based on a linearoperating range of the amplifier.
 24. The method of claim 13, whereinelectronics preceding excision are configured to remain in their linearranges for the highest anticipated interference level.
 25. The system ofclaim 1, further comprising an amplifier configured to receive thedigitized group of signals output by the inverse Fourier transformcircuit, wherein the fixed, predetermined threshold is based on a linearoperating range of the amplifier.
 26. The system of claim 1, whereinelectronics preceding excision are configured to remain in their linearranges for the highest anticipated interference level.